1. Field of the Invention
The present invention relates to technology for relaying a transmit signal, which was received from a transmitter, to a receiver without interference with echo signals by canceling the echo signals.
2. Description of the Related Art
A relay system increases the possible communication distance and improves reception performance using a relay.
Such relays are classified into Amplify-and-Forward (AF) type, which amplify received transmit signals without change and resend the amplified signals, and Decode-and-Forward (DF) type, which decode received transmit signals, eliminate thermal noise from the signals, and resend the thermal noise-free signals.
Meanwhile, when the relay amplifies the received transmit signal and resends the amplified signal to the transmitter, a receive antenna of the relay also receives undesired echo signals. The echo signals cause interference with the transmit signal to be resent to the receiver.
Recently, research into technology for canceling echo signals causing interference, that is, an Interference Cancellation System (ICS), has been conducted.
As shown in FIG. 1, such an ICS has a Single-Input Single-Output (SISO) structure, and includes a Source Station (SS), a Relay Station (RS) and a Destination Station (DS).
An interference cancellation method of the ICS will be described below in more detail. First, the receive antenna of the RS receives a signal to be sent to the DS from the SS, and the transmit antenna of the RS amplifies the signal to be sent to the DS and resends the amplified signal to the DS. In this case, the signal that is amplified or processed by the replay is called a retransmit signal. During such a process, the receive antenna of the RS receives the retransmit signal resent from the transmit antenna of the RS to the DS via the echo channel. This signal is called an echo signal. The echo signal is received from the RS and interferes with the transmit signal to be resent to the DS.
Therefore, the amplification strength of the transmit signal to be resent to the DS is restricted in the transmit antenna of the RS in order to reduce the influence of the echo signal received at the receive antenna of the RS.
However, the installation of the filter for canceling echo signals generated in spite of the restriction in the amplification strength considerably complicates the receiver structure of the RS and does not guarantee the stable retransmission and reception of signals. That is, the continuous overlap and amplification of the remaining echo signals that are not completely cancelled may cause a saturation of echo signal in the retransmitted signal, which should be free from the echo signal.